The Coalition announced last week that, as part of its plans to introduce a £144-a-week "single-tier" state pension, workers would need 35 years of National Insurance contributions (NICs) to qualify for the full pension, compared with 30 years at present.

But in letters received as recently as last weekend – after the single-tier announcement – some people approaching retirement were being reassured that they would need just 30 years' NICs because the requirement had "been reduced" – a reference to a previous reduction in the number of years required.

The Government said it would not be writing again to recipients of the letter to clarify that they would indeed need 35 years' NICs if they reached state pension age after the introduction of the single-tier or "flat-rate" pension, expected in 2017.

One leading campaigner said the letters were "misleading" and that their arrival at about the same time as the announcement of the 35-year requirement would cause "confusion".

The letters were sent to tell people that, under separate legislation already passed, their state pension age would be rising. Many of the recipients are likely to be women approaching retirement, who will see their state pension age rise to 65 or 66, depending on their exact date of birth. Crucially, many will reach state pension age after 2017, so they will be subject to the 35-year requirement if the flat-rate scheme is introduced on schedule.

But after telling recipients what their own state pension age would be, the letters went on to tell them that the requirement for NICs was just 30 years. No mention was made of a possible change to this requirement coinciding with the single-tier scheme.

The letter said: "On top of the increase in women's state pension age, other changes have made state pensions more widely available ... The number of 'qualifying years' of paid or credited National Insurance contributions you need for a full basic state pension has been reduced to 30 years."

This reduction took place in April 2010. Before then, men reaching state pension age required 44 years of NICs, while women needed 39.

A spokesman for the Department for Work & Pensions said it was unable to amend the letter until the White Paper had been published. But she added that the letter would now be changed to refer to the planned increase in NIC requirements.

However, those who had received the original version of the letter, referring to the reduction to 30 years, would not be sent another letter informing them of the latest proposals, she said. Instead, the Government would rely on people finding out through the media or other channels.

"We have a requirement to tell people about changes to their state pension age. All future letters will make reference to the proposal launched last week to introduce a single tier state pension," the spokesman said. "We are making sure people know how the new proposal will affect them through a variety of communications, our website and future pensions service."

Ros Altmann, a former government pensions adviser, said the letter was "misleading" and that recipients "just wouldn't realise" that it had been superseded by the proposals in the White Paper.

"The letter should have mentioned the possibility that the rules would change," she said. "What's the point of sending out a letter that's clearly going to be wrong? It made it sound like the reduction to 30 years had just taken place and was good news.

"I'm concerned that people are going to be misled. It's very unfortunate timing. If people received their letter after the White Paper was published, they might think it overrides it. The letter gives a personalised state pension age, so recipients might think the mention of 30 years' NICs is their own requirement."

Ms Altmann also pointed out that the letters were still being sent out 13 months after the measures increasing the state pension age became law, meaning that some people were receiving very little official notice that they would have to work longer before receiving their pension.